On an extraordinarily high strength of a two-phase VT8-based titanium alloy heavy alloyed with zirconium

R.A. Gaisin, V.M. Imayev, E.R. Gaisina, R.A. Shaimardanov, R.M. Imayev show affiliations and emails
Received: 21 October 2016; Revised: 31 October 2016; Accepted: 31 October 2016
This paper is written in Russian
Citation: R.A. Gaisin, V.M. Imayev, E.R. Gaisina, R.A. Shaimardanov, R.M. Imayev. On an extraordinarily high strength of a two-phase VT8-based titanium alloy heavy alloyed with zirconium. Lett. Mater., 2016, 6(4) 327-332
BibTex   https://doi.org/10.22226/2410-3535-2016-4-327-332

Abstract

New two-phase VT8-based titanium alloy  and its mechanical properties are considered. Microstructure of the VT8-20Zr-0.1B alloy after multiple isothermal forging (a) subsequent quenching (b) and ageing (c) is shown on the figure. A novel two-phase titanium alloy based on VT8 having the composition VT8-20Zr-0.1B (Ti-6.5Al-3.3Mo-0.3Si) (wt. %) has been considered in the present paper. It has been established that modifying with boron leads to refinement of prior β grains and α/β colonies by a factor of about ten and alloying with zirconium results in some additional colony refinement. Alloying with 20 wt. % of zirconium leads to decreasing the α+β→β transformation temperature by about 100°C that along with refinement of the as-cast structure improved the hot workability of the alloy. After multidirectional hot forging in the α+β phase field (Т = 800°С) and hardening heat treatment the strength of the ВТ8-20Zr-0.1B alloy at 20-500°C was found to be higher by 30-40% as compared with that of the VT8 alloy after the same treatment while retaining similar ductility of the alloys. This allows one to characterize the VT8-20Zr-0.1B alloy as an ultrastrong titanium alloy. For the VT8-20Zr-0.1B alloy the following tensile properties have been attained: σUTS = 1560 MPa and δ = 4% at room temperature, σUTS = 1230 MPa and δ = 14% at Т = 500°С. The specific strength reached at Т=500°С (σUTS/ρ ≈ 248 MPa/g×cm-3) is the highest even obtained at this temperature for currently known titanium alloys. It was revealed that water quenching from the β phase field led to the martensitic β→α″ transformation with retained β phase and subsequent ageing resulted in formation of ultrafine lamellar structure with a nanosized lamellae thickness that promoted achieving significant strengthening.

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